How reliable is DNA profile matching?
When two randomly chosen DNA samples match completely in a large number of regions, such as the 13 used in the FBI's system, the probability that they could have come from two unrelated people is very small.

This fact makes DNA identification extremely reliable (when performed correctly). The degree of reliability is generally measured by using the product rule of probability theory to determine the likelihood of finding a particular profile among a random selection of the population.
For example, consider a profile based on just three sites. The probability that someone would match a random DNA sample at any one site is roughly one in ten (1/10). So the probability that someone would match a random sample at three sites would be about one in a thousand:

In 2005, an actual analysis of the Arizona database uncovered 144 individuals whose DNA profiles matched at nine loci. There were another few that matched at ten loci, one pair that matched at eleven, and one pair that matched at twelve.

The eleven and twelve loci matches turned out to be siblings, hence not random. But the others were not, and were, in fact, close to what one should expect from the mathematics when you replace our simplifying 1/10 single-locus match assumption with a realistic figure obtained empirically.

Q: I BELIEVE YOU SAID THAT YOU ONLY TESTED NINE OF THE THIRTEEN MARKERS ON THESE VARIOUS SAMPLES. IS THAT RIGHT?
A: THAT IS CORRECT.

Q: WHY NOT ALL THIRTEEN?
A: ONCE I OBTAINED A MATCH WITH THE DEREK VAN DAM PROFILE, THERE WAS NO ADDED OR MORE USEFUL INFORMATION I COULD HAVE OBTAINED BY ADDING THE ADDITIONAL GENETIC MARKERS IN THE CASE.
(Perhaps if 10 or more markers were tested, the match might be from a different family member, not Derek.)

Q: IS THAT BECAUSE THIS WAS AN ITEM OF EVIDENCE TO YOUR KNOWLEDGE THAT WAS ACTUALLY FOUND INSIDE THE VAN DAM HOME?
A: THAT'S CORRECT.
(If it's DNA and it's found in the van Dam home, it must be from a van Dam family member? Bullshiit!!)

Thanks, DTTJ, that's a really good find. That article is invaluable from two points of view. One is that it was written by a prominent statistician making it authoritative; the other is that it includes some actual figures from a real database. This confirms my intuitive feeling that we should distrust those astronomical numbers. Whenever a DNA match is claimed, the number of loci it is based on should also be specified - and the number of samples in that database. But we mustn't go too far in the opposite direction. If the match is based on 13 loci, then there is a strong possibility that you have correctly identified the person - but not quite 100% certainty.

LINCOLN, Neb. -- A relatively new form of DNA testing could result in the first exoneration under Nebraska's four-year-old DNA testing law.

Juneal Pratt is serving 32 to 90 years in prison for the rape, sexual assault and robbery of two sisters at an Omaha motel in 1975.

His lawyers say that recent DNA tests done on clothing worn by the women during the attacks excluded Pratt as the perpetrator.

The DNA analysis, done at the University of Nebraska Medical Center, is known as Y-STR testing. It relies on genetic material found in the Y chromosome found only in men and is most often used in paternity cases.

One of its first uses in a high-profile criminal case came in the successful prosecution of David Westerfield for the 2001 murder of 7-year-old Danielle van Dam in San Diego.